A Swiss flag is pictured on the Federal Palace (Bundeshaus) in Bern, Switzerland December 7, 2018. REUTERS/Denis Balibouse

ZURICH (Reuters) - The Swiss government will split off its purchase of new fighter jets from its order of new surface-to-air defenses, it said on Thursday, setting a budget of 6 billion Swiss francs ($5.96 billion) for the jets alone.

The neutral Alpine country had previously agreed to spend up to 8 billion francs for a combined package under its Air2030 program, but has now divided the plan so that voters could decide separately on buying new jets in a likely referendum.

European aerospace group Airbus, France’s Dassault, Sweden’s Saab, and Boeing and Lockheed Martin from the United States submitted bids in January to replace the aging Swiss fighter fleet.

Switzerland’s stable of Boeing McDonnell Douglas F/A-18C and D Hornets and Northrop F-5 Tigers is scheduled to be retired within years. Airbus’s Eurofighter, Boeing’s F/A-18 Super Hornet, Dassault’s Rafale, F-35As made by Lockheed Martin and Saab’s Gripen E are in the running.

“It is clear to the government that public expectations to decide on the procurement by referendum pertain only to the fighter jets,” the cabinet said while asking the defense ministry to draw up a plan for this by early September.

The two weapons systems remain interlinked, however, so the deals should proceed in parallel, it added. It also decided that offset deals should cover 60 percent of the purchase amount rather than 100 percent as first planned.

Assessments of the aircraft will continue through 2020 before the government decided on a replacement, with new jets to be delivered by 2025.

Defense procurement agency Armasuisse asked manufacturers to submit pricing for 30 or 40 planes, including logistics and guided missiles, among other criteria for the bids.

Switzerland, which last fought a short war in 1847, has struggled in the past to convince citizens to back a deal for new fighters. In 2014, around 52 percent voted against a 3.5 billion franc plan to buy 22 Gripen fighter jets from Saab.